Method of preventing surface flaws on steel strip produced in pre-treatment furnace of continuous hot-dipping process

ABSTRACT

In a continuous hot-dip metal coating process into which a heattreating furnace, e.g. non-oxidizing type or Sendzimir type is included, surface flaws of traveling strip produced by deposits of metallic oxides on hearth rolls of said furnace can be prevented by cooling the hearth rolls, preferably at less than 200*.

United States Patent 1191 Hoshino et al.

[ Sept. 23, 1975 METHOD OF PREVENTING SURFACE FLAWS 0N STEEL STRIP PRODUCED IN [56] References Cited PRE-TREATMENT FURNACE OF UNITED STATES PATENTS CONTINUOUS HOT-DIPPING PROCESS 2,323,305 7/1943 Burruss et al 432/235 Inventors: Yoshio Hoshino, Tokyo; Masahiko 356L131 2/1971 165/90 N k T k M b th 3,728,144 4/1973 ll7/ll4R aga a es 0 3,802,495 4/1974 Hordes 165/89 of Yokohama, all of Japan Assignee: Nippon Kokan Kabushiki Kaisha, Primary Examiner-Cameron K. Weiffenbach Tokyo, Japan Assistant Examiner.Edith R Buffalow lk Filed: June 1973 Attorney, Agent, or Fzrm George B Ou evo [5 7] ABSTRACT In a continuous hot-dip metal coating process into which a heat-treating furnace, e.g. non-oxidizing type Appl. No.: 374,342

US. Cl. 423/235; 432/198; 427/355;

427/398; 165/90 or Sendzimir type is included, surface flaws of travel- Int. Cl? F27D 3/00; c230 1/00 ing StiP Pmdmed by dePFSitS metallic ides Field of Search l 17/] 14 R, 1 14 A l 14 C; hearth rolls of said furnace can be prevented by cooll65/89 90; 432/235 198; 339/1l2;427/355, ing the hearth rolls, preferably at less than 200.

360, 398 3 Claims, 1 Drawing Figure III Water flow Water 1 low METHOD OF PREVENTING SURFACE FLAWS ON STEEL STRIP PRODUCED IN PRE-TREATMENT FURNACE OF CONTINUOUS HOT-DIPPING PROCESS BACKGROUND OF THE INVENTION The present invention concerns a method of preventing surface flaws on a steel strip which would be produced in a furnace of a hot-dip metal coating process, and more particularly to the method of preventing surface flaws on the traveling strip which are produced by deposits of metallic oxides on hearth rolls of a heat treating furnace of the non-oxidizing type, Sendzimir type, or others.

BRIEF DESCRIPTION OF THE PRIOR ART Today, the usual practice in the art is to give a heat treatment to the steel strip passing through continuous I hot-dip metal coating process facilities. However, a steel strip passing through the line tends to gather such extraneous substances as metallic oxides on hearth rolls of a heat-treating furnace, which are brought in by a previous traveling strip, irrespective of the type of said furnace. These substances are fine particles, but they naturally grow bigger to form a mass as they are sintered on the hearth rolls and become a deposit.

Generally, sintering is subject to types of metals, metallic oxides, size of the particles and atmosphere of furnaces. In the pre-treatment in the continuous hotdip. galvanizing facilities, the furnace temperature is quite high, e.g. 1,000l,200 C in the non-oxidizing furnace and 600l ,000 C in reducing furnaces, and is within the sintering temperature of iron oxide. Said sintering is particularly remarkable in a steel strip which has been subjected to oxidation in the furnace and which has a fine oxidized film on the surface. As the atmosphere in the furnace is either reducing or slightly acid, iron is most likely to be sintered in such an atmosphere.

In the conventional art, the sintering phenomenon of iron oxide is a function of time and temperature, and

the temperature at which the starting of sintering is held to be within 600 C650 C. According to the experiences we have had, the size of extraneous substance sticking to the hearth roll in the foregoing process which grow larger is about 10-20 mm in height and becomes a deposit of conical shape. When the steel strip passing through the line comes into contact with such deposits, a dent of convex shape is formed on said strip and remains even after finishing. This naturally injures the appearance of the products.

Various methods have been proposed and practiced to prevent accumulation of such extraneous substances on the hearth rolls, as follows.

l. Lowering the temperature in the furnace, so that said extraneous substances are held back from sticking and growing.

2. Giving higher tensile strength to the steel strip passing through the furnace, the contact pressure of the hearth rolls and the steel strip being lowered.

3. The peripheral speed of the hearth roll is made to match the travelling speed of the strip as much as possible.

4. The material of the hearth roll is examined.

Although the above-mentioned methods decrease or hold back the sticking of the said extraneous substances or the growth thereof to some extent, they do not prevent the same to a satisfactory degree. Rather,

the production of said deposit tends to increasedue to the operations conducted at a higher furnace temperature for further improvement of theproduction efficiency. The recent trend has resulted in a high deposit after only about two weeks whichrequir'e the repair of the hearth rolls or operation at a lower temperature, and in extreme cases the replacement of the rolls had to be performed in l or 2 months. Naturally, the products manufactured during such a period were with inferior surfaces. Since the sticking strength of said extraneous substance to the hearth roll is strong, the operation must be stopped fully and the deposit must be removed by hand work or the hearth rolls must be replaced wholly. This is mainly the cause for lowered operational efficiency, low productivity and inferior surface quality of the strip. Unfortunately, there has not been found any suitable or effective means to obviate these defects in the art.

The present invention was developed with a view to break through such a situation, and is characterized by the fact that the hearth rolls provided in the heating furnace are of the water cooled type. If the surface temperature of the water cooled type rolls was maintained below the maximum of 200 C, then the sticking of extraneous substances such as iron oxides was completely prevented. This will further eliminate the suspension of the operation for repair of rolls as well as the lowering of efficiency based on operation at a lower furnace temperature.

An object of this invention is to provide an improved method of preventing extraneous substances, brought into pre-treatment furnace of continuous hot-dip metal coating process furnace from sticking tohearth rolls of said furnace and the growth into adeposit.

Another object of this invention is to provide an improved method of preventing surface flaws in a steel strip workpiece from being produced in a pretreatment stage without lowering its productivity.

Other objects and advantages of this invention will be apparent from the following description taken together with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING The attached drawing is an explanatory view of a hearth roll employed in the treatment of steel strip material, according to the inventive concept.

DETAILED DESCRIPTION The water cooled type rolls used in the embodiment of the present invention are manufactured in a very simple manner.

In describing the present invention more in detail with reference being made to the drawing, the roll axis 1 is hollow. A jacket 4 is formed between the roll body 2 which is also a hollow cylinder and the said roll axis 1, and the jacket 4 is formed with a spiral dashboard.

Cooling water is fed from one end of the roll axis 1 passes as it cools the roll axis 1 and enters the jacket 4 at the entrance of the roll body 2.

Generally speaking, the present invention contemplates an improvement in the continuous hot-dip metal coating process into which a heat-treating furnace, e.g. non-oxidizing type or Sendzimir type is included, surface flaws of a traveling strip produced by deposits of metallic oxides on hearth rolls of said furnace can be prevented by cooling the hearth rolls, preferably at less than 200 C.

The stream of water rotates spirally by the dashboard 3 provided within the jacket 4 and cools the periphery of the cylindrical roll body 2 uniformly. At the other end of the roll body 2, the cooling water again enters the roll axis 1 and then is discharged from the other end of the roll axis. In such a construction, the water stream fed by the spiral dashboard and with centrifugal force based on the rotation of rolls always flows along the periphery of rolls and has an extremely big cooling capaclty.

The water cooling type rolls used in the present invention is very simple and may be varied limitlessly. However, what is mosLsignificant is cooling capacity of the periphery of the cooling type rolls which come into contact with the travelling steel strip. If this was to be made from existing configurations, then any type of cooling type rolls may be utilized for the present invention purposes.

According to the experiments carried out by the inventors of this invention, the surface temperature of the rolls on the furnace floor should be 200 C at highest. If the temperature exceeds 200 C, the sticking of extraneous substances as iron oxides begin to be seen and it has been confirmed that it is no longer effective.

The following are the embodiments of the present invention utilizing the cooling type rolls as shown in the attached drawing.

EMBODIMENT Furnace used: Non-oxidizing furnace (continuous hot-dip metal coating) Hearth rolls of said furnace.

Furnace temperature: l,lOOl,250 C Furnace atmosphere: CO 7%, CO O tr, H 10%, remainder H O, N Cl-l Hearth rolls: water cooling type (as in drawing) Material: Boiler Tube in ASTM Cooling water: 60 I/min Surface temperature: Approximately 100C Results: No extraneous substances found sticking on the hearth roll after continuous operation for 3 months EMBODIMENT 2 Furnace used: Sendzimir type (continuous hot-dip metal coating) Reducing furnace hearth roll Furnace temperature: 900950 C Furnace atmosphere: H N 25%, dew point Ol0C Hearth rolls: Same as in Embodiment 1 Results: No extraneous substances found sticking on the hearth roll after continuous operation for 3 months In the above-mentioned Embodiments, results after 3 months continuous operation were cited. This does not mean that the operation became impossible at this stage, but was merely suspended to confirm the results. The operation naturally was resumed thereafter.

As has been discussed hereinbefore in detail, the cooling water passes along the periphery of the rolls constantly, and the cooling effects are extremely high and uniform and there is no air pool formed within the roll and the uniform thermal stress of the rolls could also be achieved. Therefore, there is no deformation of the rolls caused by the thermal strain. Since there is no sticking of extraneous substances to the rolls if the operation is performed in accordance with the present invention, there is no longer the lowering of operational efficiency caused by the suspension of the operation. The most remarkable advantage of the present invention lies in the fact that the surface flaws of the product have been completely eliminated and that it brings about a most valuable practical value.

We claim:

1. In a continuous hot-dip metal coating process, a method of preventing surface flaws on a traveling steel strip workpiece caused by deposits of metallic oxides on hearth rolls within a heat treating furnace formed by previous workpieces traveling through the hearth rolls comprising cooling said hearth rolls with a cooling fluid during said heat-treating so that the temperature of said hearth rolls do not exceed 200 C. by feeding a stream of water through the roll axis and spirally rotating said stream of water through the inside of said hearth rolls so that the water cools the periphery of said hearth rolls to said maximum temperature of 200 C.

2. A method as set forth in claim 1 wherein said heat treating furnace is a non-oxidizing type furnace.

3. A method as set forth in claim 1 wherein said heat treating furnace is a reducing type furnace.

l l =l 

1. IN A CONTINUOUS HOT-DIP METAL PROCESS, A METHOD OF PREVENTING SURFACE FLAWS ON A TRAVELING STEEL STRIP WORKPIECE CAUSED BY DEPOSITS OF METALLIC OZIDES ON HEARTH ROLLS WITHIN A HEAT TREATING FURNACE FORMED BY PREVIOUS WORKPIECES TRAVELLING THROUGH THE HEARTH ROLLS COMPRISING COOLING SAID HEARTH ROLLS WITH A COOLING FLUID DURING SAID HEAT-TREATING SO THAT THE TEMPERATURE OF SAID HEARTH ROLLS DO NOT EXCEED 200*C. BY FEEDING A STREAM OF WATER THROUGH THE ROLL AXIS AND SPIRALLY ROTATING SAID STEAM OF WATER THROUGH THE INSIDE OF SAID HEARTH ROLLS SO THAT THE WATER COOLS THE PERIPHERY OF SAID HEARTH ROLLS TO SAID MAXIMUM TEMPERATURE OF 200*C.
 2. A method as set forth in claim 1 wherein said heat treating furnace is a non-oxidizing type furnace.
 3. A method as set forth in claim 1 wherein said heat treating furnace is a reducing type furnace. 